30 
HINTS TO TRAVELLERS. 
either of the joints, must find its way. The lower or reservoir end 
(Fig. 8) is about 4*5 inches long, and has an air-tight glass tap about 
an inch below the broad part. These ends are forced into the rubber 
tubing, and, as an additional precaution against leakage, copper wire is 
bound round the joints. The scale is cut on an aluminium bar, along 
which two carriages, to which the barometer is attached, move up and 
down, and they can be clamped to the bar at any place (Fig. 6). By 
means of the verniers attached to the carriages, which are divided to 
0‘002 of an inch, it is easy to estimate the height of the mercury to O’OOl. 
To use the barometer, the carriages are put on the scale bar; the lower 
one is clamped at the bottom of the bar, and the upper one some inches 
higher up; the barometer is attached to the carriages by clamps which 
fit over the joints ; the rubber cap is removed from the reservoir end, the 
tap opened, the verniers put in the middle of their runs, and the upper 
carriage moved up the bar until there is a vacuum. By means of the 
screws on the right of the mercury, the verniers are moved up or down 
until the top of the mercury at each end is in line with the edges of the 
rings attached to the verniers, which fit round the glass ends. Both 
verniers are then read, and the difference gives the height of the baro¬ 
meter. The rubber cap on the reservoir end is merely to prevent the 
small quantity of mercury, which should be left above the tap when it 
is closed, from being shaken out when travelling. 
To pack up the barometer, lower the upper carriage very slowly until 
the mercury has touched the top of the glass; then detach the barometer 
from this carriage, and either let the upper end hang vertically below the 
reservoir, or detach the reservoir end from its carriage and raise it till 
the barometer hangs vertically. By this means the barometer is com¬ 
pletely filled with mercury, and then the tap must be closed. The tube 
is then to be coiled away in its padded box. When too much air is 
found in the trap, it must be extracted by means of the air-pump. 
The Aneroid Barometer .—The aneroid barometer is so convenient on 
account of its portability that, although much less reliable than a. 
mercurial barometer, it is much more likely to be used by a traveller.- 
Care should be taken in using it to see that the pointer has come to a 
position of equilibrium, and it should be tapped gently before reading' 
The eye must be brought directly over the end of the pointer, and the 
reading made to one-hundredth of an inch, the barometer being held in 
